DC to DC converters with a transformer are configured to completely isolate between input and output thereof, and therefore, they have been widely used.
In such DC to DC converters, DC to DC converters with a pair of transformers have been well-known, some of which are disclosed in U.S. Pat. No. 7,254,046 B2 corresponding to Japanese Unexamined Patent Publications No. 2005-51994 and No. 2005-51995.
In the US patent Publication, one type of DC to DC converters is provided with a first converting circuit between an input DC power source and a pair of first and second transformer assemblies 15 and 16, and with a second converting circuit between a load and the pair of first and second transformer assemblies T1 and T2.
The first transformer assembly T1 includes a core, a first primary winding W1, a second primary winding W2, and a first secondary winding W3. The second transformer assembly T2 includes a core, a third primary winding W4, a fourth primary winding W5, and a second secondary winding W6.
The first converting circuit includes a first switching element Q1 and a second switching element Q2, which are controlled to be complementarily turned on and off. Similarly, the second converting circuit includes a third switching element Q3 and a fourth switching element Q4, which are controlled to be complementarily turned on and off.
The complementary switchings of the first and second switching elements Q1 and Q2 and those of the third and fourth switching elements Q3 and Q4 allow the first and second transformers T1 and T2 to operate alternately as a transformer and a reactor.
In the US patent Publication, another type of DC to DC converters is disclosed. In another type of DC to DC converters, paired first and second transformer assemblies T1 and T2 are structurally integral with each other so as to at least partially have an integral core at least part of which serves as a common magnetic path therebetween. The integral structure of the first and second transformer assemblies T1 and T2 can reduce its total size. The integral structure of DC to DC converters having a common magnetic path between paired first and second transformer assemblies will be referred to as “common magnetic-path structure” hereinafter.
The first to fourth switching elements Q1 to Q4 are individually turned on and off at a high frequency so that high frequency switching currents caused by the high frequency switchings are adapted to flow through the first and second transformer assemblies. For this reason, ferrite-powder cores or the like, which allow such a high-frequency switching current to flow therethrough, are preferably used as the cores of the first and second transformers T1 and T2.